In a nutshell: Although it may seem counterintuitive, our perception of low-contrast images can be improved by adding noise to the images themselves or directly to the brain.

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We generally think of random noise as an unwanted interference, obscuring a signal that we want to receive. And in some cases, that’s true: adding superfluous information can make it more difficult to detect the information of interest.

But in other cases, adding noise can actually help to boost a signal that would otherwise be too weak to detect. This is due to a phenomenon called ‘stochastic resonance’ (SR).

Noise contains many different frequencies all mixed together. Think of the static displayed on old analogue televisions when no signal was received, or the sound of a non-existent radio station. In SR, the frequency in the noise that matches the weak signal helps to strengthen it, boosting the signal-to-noise ratio and enabling the signal to be detected.

In humans, various brain activities can be affected by SR. For example, random noise is believed to affect binocular rivalry. When two different images are presented to our eyes at the same time – one to the left and one to the right – we don’t see both images superimposed. Instead, our brain switches our visual awareness spontaneously between the two images. We see either one image at a time (exclusive perception) or, during the transitions between images, an unstable composite of the two (mixed perception).

To study this further, Onno van der Groen and Nicole Wenderoth from ETH Zurich and Jason Mattingley, a Brain Function CoE investigator from the Queensland Brain Institute, ran two experiments in which participants performed tests involving binocular rivalry. In each case, the participants viewed low-contrast or high-contrast visual stimuli on a computer screen.

In the first experiment, the researchers added noise to the visual stimuli. In the second experiment, they used transcranial random noise stimulation (tRNS) to add noise directly to the participants’ visual cortex – the part of the brain involved in processing visual information.

The researchers found that adding noise during binocular rivalry significantly affected participants’ perception in both experiments – but only for low-contrast images. The added noise reduced the duration of mixed perception by up to 16%, allowing the brain to switch more quickly between periods of exclusive perception. No similar effect was found with the high-contrast images.

The researchers now have more information about how noise affects brain activity, which could help them to understand conditions in which the brain responds differently to binocular rivalry. Studies have shown that in people with autism, for example, periods of mixed perception during binocular rivalry last longer than in people without autism.

This study also confirms that the visual cortex is involved in binocular rivalry, which was previously a matter of debate.

Next steps:
The researchers are looking for evidence of SR in the part of the nervous system that controls movement, by combining tRNS with another form of non-invasive brain stimulation called transcranial magnetic stimulation. They are also planning to use SR to improve recovery in patients after brain injury such as stroke.

van der Groen, O., Mattingley, J. B., & Wenderoth, N. (2019). Altering brain dynamics with transcranial random noise stimulation. Scientific Reports, 9(1), 4029. doi: 10.1038/s41598-019-40335-w

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Rajiv Madipakkam, A., Ludwig, K., Rothkirch, M., & Hesselmann, G. (2015). Now you see it, now you don’t: Interacting with invisible objects. Frontiers for Young Minds, 3, 4. doi: 10.3389/frym.2015.00004. Published under a Creative Commons Attribution license.